1. Field of the Invention
The present invention relates to a semiconductor device and a method of making the same, and more particularly to improvements to the base electrode of an Heterojunction Bipolar Transistor (HBT).
2. Description of the Related Art
Bipolar transistors play an important role as a high-speed integrated circuit device because of their capability of outputting a large load drive force. In order to operate a bipolar transistor at high speed with low power consumption, it is effective to increase the cut-off frequency and to reduce the quantity of electric charges stored in the transistor. To this end, the base layer must be thin so as to reduce the electron transmit time.
In conventional silicon transistors, the emitter impurity concentration is required to be higher than the base impurity concentration to improve the emitter injection efficiency. However, as the base is thinned, a punch-through breakdown voltage is lowered. To obtain a high punch-through breakdown voltage, the base concentration is required to increase. As a result, the emitter/base junction becomes a high impurity concentration pn junction, in which a band-to-band tunnel current is tend to increase, which hinders a normal transistor operation. When the collector-emitter breakdown voltage is kept at about 3 volts in the silicon transistor, the lower limit of the base thickness is about 50 nm and the upper limit of the corresponding cut-off frequency is about 50 GHz considering the above-discussed situations.
To make the base layer thinner and to make the cut-off frequency higher over these limits, a hetero-emitter structure in which the forbidden band width of the emitter is larger than that of the base is attracting attention. In the hereto-emitter structure, since the injection of, for example, positive holes from the base of an npn transistor to its emitter is suppressed due to the difference of widths between the forbidden bands of the base and emitter, the injection efficiency is not lowered even if the emitter concentration is lowered below the base concentration. Therefore, the emitter and base concentrations can be determined independently from the injection efficiency, so that the base concentration can be increased enormously compared to the conventional homojunction transistors. As a result, the base thickness can be reduced and therefore the cut-off frequency can be made higher.
In order to realize such heterojunction bipolar transistor, it is required to coincide the position of the heterojunction with the position of the pn junction between the emitter and base with the accuracy in the order of 1 nm. Further, it is required to increase the base concentration in order to reduce the base thickness. However, it is very difficult for the existing technology to satisfy these two requirements at the same time. This is because, in the fabrication of bipolar transistors for example, diffusion techniques are employed to form the emitter and the base as well as to form the high concentration layer for the leading electrode. During high temperature treatment in the diffusion process, highly concentrated impurities doped into the base layer are likely to diffuse, which deviates the position of the pn junction from the position of the heterojunction.
In order to realize higher speed bipolar transistors, the base layer must be as thin as in the order of 10 nm. Therefore, it has been a problem to be solved how to form the high concentration layer to lead the base electrode.
As discussed above, the conventional HBT has a problem that highly concentrated impurities doped into the base layer are diffused in the high temperature treatment of the diffusion process when forming the high concentration layer for leading the base electrode, and therefore the position of the on junction deviates from the position of the heterojunction.